It sounds as though you are fortunate to have an open-minded principal! Believe me, that is a big part of the solution. As you read and learn more about options, you may find other ideas that can be implemented in her current school. For example, she could take math or other subjects with an older class. She might be provided higher level books (appropriate level) or spelling words during class time.

Might there be the possibility of your nanny returning to homeschool her?